The disclosure is based on a sensor arrangement for indirect detection of a torque of a rotatably mounted shaft.
Sensors for detecting torque represent important components of engine and transmission test benches of all types. Moreover, they are an integral constituent part of many drive systems used in large-scale industry. With their aid, for example, the torques in ships' shafts, wind turbine systems or drilling rigs are monitored. Torque sensors are widespread but their comparatively complex structure and the costs associated therewith have previously prevented their use in mass-produced products. The measurement of the torque of the drive shaft of electric bicycles represents the first potential mass-market for torque sensors but the sensor concepts used for industrial systems are too expensive for this purpose.
In many test benches or calibration devices, the detection of the torque by means of a static transducer is sufficient. Here, the shaft whose torque is to be detected is connected to one side of a deformation element. The other end of the deformation element, also called a spring element, is connected to a stationary constructional element, such as a carrier or a housing part, for example. The applied torque leads to deformation of the spring element as a result of torsion. The resultant twisting is of a few degrees and can be detected by a multiplicity of known measuring methods. Here, above all, magnetic methods which detect the twist of a magnetic structure attached to the spring element relative to a stationary magnetic field sensor are usual. Optical methods are also suitable for this purpose.
Alternatively, it is possible to detect the material strains arising in the spring element on account of the deformation. Depending on the construction, these result from torsional torques or shear forces. To measure these material strains, bonded-on piezoresistive strain gauges, which are wired up to form a Wheatstone bridge, are generally used. Alternatively, these strains can also be detected by means of the magnetoelastic measuring principle. This is based on the fact that the permeability of ferromagnetic materials changes when material stresses are introduced. These changes can be detected without contact by a suitable sensor system, for example in the form of a structure comprising transmitter and receiver coils.
In most applications, the above-described static detection of the torque is not sufficient. Instead, it is necessary to determine the torque of rotating shafts. For this purpose, co-rotating sensors have been developed, which are integrated into the drive shaft and measure its torsion. This is normally carried out via one of the two above-described methods for determining the material strains caused by the torsion.
When strain gauges are used, the problem arises that neither the supply to the measuring bridge nor the signal pickup can be carried out via a cable connection in a rotating system. The supply is normally effected by the transmission of an alternating voltage by means of a transformer arrangement, in which a coil is wound around the drive shaft and consequently co-rotates. The other coil is stationary and surrounds the shaft at a somewhat greater distance. Together with the shaft, which acts as an iron core, the result is thus a transformer with comparatively good characteristics. Since the output signals of strain gauge bridges are relatively small, the signal evaluation and amplification are therefore carried out in the immediate vicinity of the measuring bridge by means of co-rotating electronics. The output signal from the latter can then, for example, be transmitted to the outside, i.e. to the stationary part of the sensor, by a transmitter coil and a receiver coil or by further electronics by means of a radio standard. Such sensors and all the components needed therefor are known from the prior art. They meet the requirements placed upon them but, as already explained above, require a high constructional outlay. In the co-rotating torque sensor area, magnetoelastic sensors have inherent advantages, since the measuring method used is non-contacting. The problem of making contact with rotating components is therefore not posed at all here, which is reflected in a lower constructional outlay.
Both with piezoresistive and with magnetoelastic sensors, the torques on rotating shafts can be measured very well. Their greatest advantage is the direct measuring principle. The torsion of the shaft detected by said sensors has a direct relationship with the torque. However, their greatest disadvantage also derives from this. The properties of shaft and sensor are inextricably connected with each other. The sensors cannot be applied to an existing shaft, since the elastic and/or magnetic properties of the shaft dominate the sensor characteristics. Instead, the torque sensors are themselves part of the shaft. Their specific requirements therefore have to be taken into account from the start during the construction of the entire drive train. A constructional solution that is found for one system cannot simply be transferred to another application. This leads to the situation in which, for example, the manufacturers of magnetoelastic sensors offer a total package comprising shaft (including all gears), integrated torque sensor and the necessary bearings. This integration represents a good solution primarily in manufacturing terms. However, it is likewise highly application-specific and can therefore be used for other applications only with difficulty or even not at all. As a result, the quantities and therefore the cost potentials for such solutions are limited by their nature.
As an alternative to direct measurement of the torque, it is possible to measure the forces arising during the transmission of the torque from one shaft to another shaft on the bearings thereof and to draw conclusions about the torque therefrom. This indirect approach is known from the prior art and is disclosed, for example, in the documents DE 10 2012 200 232 A1 and DE 10 2010 027 010 A1. However, these documents contain no kind of practical implementation with which the measurement of the bearing forces can be carried out.